While tags and groups are technically the same, their different way of presentation can be more flexible, especially in large DEVONthink knowledge collection spreading out in many directions. Using the tags bar (which you show and hide with View > Show/Hide Tags) you quickly add single tags to documents or remove them. Automatic completion makes adding already known tags easy. To select documents that have one or more tags assigned to them, either use the tag view (View > as Tags) or, even better, create a smart group (Data > New > Smart Group) that always contain all matching items completely automatically.
If you want to assign multiple tags to or remove them from a larger number of documents at once, use Scripts > Tags > Add tags to selection or Scripts > Tags > Remove tags from selection which you can install in DEVONthink Pro from Help > Support Assistant on the Extras tab.